At certain stages of their manufacture, many kinds of webs have to be coiled up for transportation to the places where they are given further treatments. Sometimes they have to be stored for long periods before the subsequent treatment takes place. Very often one or both surfaces of these webs are in such a condition that they get impaired in case they are brought into contact, especially prolonged contact or contact under great pressure.
In the manufacture of film, especially photographic film, it is common practice to physically deform the edges of the film during some stage of its manufacture in order to increase the thickness of the edges. It is known that even a slight thickening of the edges is adequate to hold apart the convolutions of the film and is sufficient to avoid the production of defects.
A further reason for edge knurling is the securance of the relative position of the windings of a roll of coiled web material so that the risk of telescoping deformation of the roll during its winding and in particular during its transportation, is very small.
In the manufacture of solvent-type webs such as cellulose triacetate film base, it is common practice to cold knurl the edges of the film at some stage which is situated between the steps of complete drying of the solidified film and winding up of the film. The film edges are conveyed between the nip of a free-rotating wheel, the peripheral surface of which is deformed in accordance with the desired knurling pattern, and a roller or wheel with a smooth peripheral surface. Depending on the surface hardness of the smooth roller or wheel the required knurling force may slightly vary but it anyway frequently occurs that the pressure between the wheels for obtaining the desired knurling must be so high that elongation of the marginal portions of the web cannot be avoided. Such elongation is easily apparent as it causes wrinkling or fluting of the film edges of the film in longitudinal direction thereof, and as such film with fluted edges is coiled up, it is noticed that the diameter of the axial end portions of the film roll is greater than the diameter of the corresponding portions of a roll of film without fluted edges.
The fluted edges of a film may cause serious troubles in the coating of the film since different coating systems, such as an air knife coater or a cascade coater comprise mechanical parts which are spaced at some tenths of a millimeter only from the web path. Another difficulty which occurs less frequently is that more film material than that corresponding with the actual knurling width must be cut away in order to obtain film without fluted edges.
Another difficulty which occurs in the treatment of cellulose triacetate films is the permanent reduction of the knurling depth of the knurled film edges. A reduction up to 50% of the original knurling depth of the film edges may occur at the first winding-up of the film and this reduction may continue, though to a lesser extent, during the successive winding-up operations of the film which take place at the consecutive steps in the manufacturing process comprising the coating of subbing, light-sensitive, anti-stress and other layers on the film. Hence, the risk of telescoping deformation of a roll of film as a consequence of insufficient edge pressure increases towards the final stage of the production cycle and, unfortunately, the economic losses run parallel with such number of stages in the production. A roll of film with insufficient edge pressure as a consequence of a decrease of the knurling depth after the film has been coiled up, is easily recognizable. Indeed, if one looks at the ends of such roll in the axial direction one may see that the cross-section of the roll has an oval rather than a circular shape under the influence of gravity.
Still another difficulty is that the initial edge knurling depth which may be required to anticipate the reduction of the knurling depth, may be so striking that the film becomes liable to transverse rupturing as a consequence of mechanical weakening of its edge portions.
In the manufacture of oriented polymeric thermoplastic webs, such as polyethylene terephthalate film, it is known to warm knurl the edges of a web by locally preheating the edge portions of the web immediately prior to the knurling operation. The preheating may be done by means of an infrared heater but in that case it is impossible to limit the heating of the web to the width of the edge portion which has to be knurled, and since the heating of the film is likely to influence the crystallinity of the film, it occurs that once the knurled edges have been trimmed, the edges of the remaining web portion are still defective. In case the preheating is done by means of heated rollers, the heating must necessarily be higher than actually required to compensate for losses during the web transport from the preheating zone to the knurling zone. Furthermore, the preheating by means of a roller occurs uniformly according to the width of the web edge portion and thus a web surface which considerably exceeds the actual surface to be knurled must be heated, which requires an increased amount of heating energy.